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Side Load Capability

A common problem with turbine actuators is oil leaking from their output shaft due to connection to

valve

rack linkages which have an arc-type of motion. This motion results in side loading of the actuator shaft,

and after long periods may result in shaft-seal wear and resultant oil leakage. Designed for a continuous

side load of up to 10% of actuator output, the VariStroke-II actuator incorporates a high-force precision

bearing and triple-seal technology on its output shaft to solve this typical application problem.

Silt Buster A patented self-cleaning feature that flushes silt and debris from the servo valve. At the interval

and amplitude selected by the user, this function  provides a very rapid motion of the servo valve spool,

allowing any silt to be flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse results in no net

change in fluid volume to the hydraulic power cylinder, and thus does not interrupt the control of the

turbine. This unique feature provides a high degree of stability, reliability, and silt resistance.

Silt Buster

A patented self-cleaning feature that flushes silt and debris from the servo valve. At the interval and

amplitude selected by the user, this function  provides a very rapid motion of the servo valve spool,

allowing any silt to be flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse results in no net

change in fluid volume to the hydraulic power cylinder, and thus does not interrupt the control of the

turbine. This unique feature provides a high degree of stability, reliability, and silt resistance.

Valve Rack Linearization (not currently available)

Because steam flow through single and staged inlet steam valves tends to be non-linear throughout their

flow range, turbine controls must be de-tuned to compensate for instability or sluggish control points

throughout this range. As a way of allowing turbine control optimization without detuning, the

VariStroke-II includes an 11-point linearization table to allow turbine OEMs or users to compensate for

poor valvelinea rization by digitally linearizing the control-to-valve flow relationship.

Side Load Capability

A common problem with turbine actuators is oil leaking from their output shaft due to connection to

valve rack linkages which have an arc-type of motion. This motion results in side loading of the actuator

shaft, and after long periods may result in shaft-seal wear and resultant oil leakage. Designed for a

continuous side load of up to 10% of actuator output, the VariStroke-II actuator incorporates a high-force

precision bearing and triple-seal technology on its output shaft to solve this typical application problem.

The VariStroke-II Actuator offers the following benefits to the user in comparison to other electro-

hydraulic actuators:

Dirt Tolerance

The VariStroke-II actuator is specifically designed for steam turbine applications where turbine lube oil is

also used to power the hydraulic turbine control valve actuator(s). Steam turbine applications can be

extremely challenging for hydraulic control valve actuators as dirt, metal shavings, water, and other

contaminants (Babbitt, ammonia, etc.) are common in such oil systems. Also due to the high

temperatures at which steam turbines operate, turbine oil breakdown is common, resulting in the creation

of a sludge-type substance and the varnishing of internal system components. However, the VariStroke-II

actuator is designed to operate reliably within such challenging applications. Its corrosion-resistant

materials, single moving rotary valve, 610 N (137 lbf) of chip shear force, and self-cleaning port design

allow it to operate in such applications without experiencing undesirable sticking or dragging.

The VariStroke-II actuator is part of a product family with many different models available for purchase

depending on the force, stroke, and redundancy needs. This actuator is available with standard bore

diameters and standard stroke ranges. The VariStroke-II’s unique “variable stroke” capability also allows

users to customize/set the actuator’s exact stroke length in the field to meet their requirement.

The VariStroke-II is factory and/or field configurable via a computer-based service tool. The actuator’s

PCI Service Tool uses a simple, user-friendly format to allow users to easily configure, calibrate, and

adjust all internal functions and response settings. The VariStroke-II also includes a 4–20 mA output

channel to indicate output shaft position, and unit alarm and shutdown relay outputs for use as unit

health and status indications.

The total installed cost for the VariStroke-II is low because it is a fully integrated actuator that has been

completely assembled and tested at the factory. This greatly reduces OEM and end-user fabrication time,

testing time, and site assembly time.

Introduction

The VariStroke-II is a linear electro-hydraulic actuator that utilizes a double-acting power cylinder with

integrated electronic driver module, servo valve, and redundant LVDT (Linear Variable Differential

Transformer) position feedback sensors to precisely control steam turbine valves. The actuator’s driver

module accepts one 4–20 mA position demand and compares it to the sensed actuator shaft position to

accurately control output shaft position.

The actuator’s output shaft position is controlled by a digital controller combined with an integrated

rotary

servo valve that ports supply oil to and from its power cylinder. The actuator digital controller architecture

allows it to perform stable position control during normal conditions, and also respond quickly to desired

valve step changes during system or plant transients. As a means of protecting the turbine, an internal

servo valve return spring forces the actuator to a failsafe position upon any internal unit failure (electrical

input power failure, position sensor failure, processor failure, etc.).

Hydraulic Power Cylinder

The VariStroke Remote Servo can be connected to any hydraulic cylinder,

however; proper operation requires that the VariStroke Stability Equation be

satisfied (see Chapter 2. Stability Specifications). In order to control cylinder

position, the Cylinder must be equipped with a position feedback sensor. The

position sensor must meet the following specifications:

 Output Signal: 4–20 mA

 Input voltage (provided by the VariStroke Circuit Board): 15 Vdc

 Update Rate: ≤1 ms

 Linearity: ±0.04% Full Stroke

 Current Drain: < 100 mA

 Sensor Length must not exceed 2 times the Cylinder Stroke Length

Cylinder Position Control

The cylinder position controller adjusts the hydraulic power cylinder position to

match the feedback signal to the demand.

Both the servo position controller and cylinder position controller are monitored to

ensure accurate tracking.

The position controller regulates a pulse width modulated (PWM) drive signal to

the actuator. The drive current to the actuator is regulated, transiently allowing up

to 10 A to be provided to move the actuator at its maximum speed and torque. A

steady-state current limit becomes active after a period of a few seconds to

protect the actuator and electronics.

The shield connections for the Analog Output (terminal #20), CAN1 (terminal

#23), and CAN2 (terminal #29) are through capacitors only as indicated in the

wiring section of this manual.

The power supply section performs the EMI filtering on the (18 to 32) V (dc) input

voltage and generates controlled voltages for several electronics sub-systems.

The power supply system is monitored for proper operation. If input voltage or

internal power systems are detected outside of allowable operating ranges, a

diagnostic alarm will be enunciated.

Calibration and configuration of alarms and shut down and redundancy operation

are configurable via the PC Service Tool.

The primary demand and redundant demand / feedback input signals are

designed for a (4 to 20) mA control signal. Each input signal is EMC protected.

Discrete outputs are provided for alarm and shut down enunciation. An internal

LED also is illuminated when a fault condition is detected. Cover needs to be

removed to see this LED. The configurable discrete output can be customized to

output a variety of enunciations using the PC Service Tool. All of the discrete

outputs are configurable for normally-open or normally-closed action using the

PC Service Tool.

Electronic Driver Module (PCB)

The printed circuit board (PCB) is mounted on top of the housing (see Figure

1-6). The PCB performs the following tasks:

 Power Supply

 Isolated Input and Outputs

 Dual Redundant Demand Inputs

 Dual Redundant Inputs for Final Cylinder Feedback

 Microprocessor-based Control

 Actuator H-Bridge Drive

 Current Limiting for Thermal Protection

 Advanced Diagnostics

 Discrete Outputs for Fault, Alarm, and Shutdown Enunciation

The shield connections for the Analog Output (terminal #20), CAN1 (terminal

#23), and CAN2 (terminal #29) are through capacitors only as indicated in the

wiring section of this manual.

The power supply section performs the EMI filtering on the (18 to 32) V (dc) input

voltage and generates controlled voltages for several electronics sub-systems.

The power supply system is monitored for proper operation. If input voltage or

internal power systems are detected outside of allowable operating ranges, a

diagnostic alarm will be enunciated.

Calibration and configuration of alarms and shut down and redundancy operation

are configurable via the PC Service Tool.

Rotary Servo Valve

The hydraulic servo valve has five ports: Supply, two Control Ports, Over Board

Drain (OVBD), and Drain/Tank. With the hydraulic valve in its middle position,

both control ports are blocked. As the valve rotates, supply pressure is

connected to a control port while simultaneously connecting the drain to the other

control port. The combined action of the servo position controller and cylinder

position controller modulate the power cylinder position as necessary to match

the input demand. OVBD is permanently connected to drain and can (optionally)

be connected to the OVBD connection on Hydraulic Power Cylinder to drain any

leakage pass the primary rod seal.

A unique function of the software is a periodic, symmetrically opposed impulse

(called “Silt Buster”) which flushes silt and debris from the servo valve without

causing undue wear. At the interval and amplitude selected by the user, this

function provides a very rapid motion of the hydraulic valve, allowing any silt to

be flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse

results in no net change in fluid volume to the controlled servo valve, and thus

does not interrupt the control of the turbine. This unique function provides a

higher degree of stability, reliability, and silt resistance.

If the unit detects any diagnostic shut down condition, or if the detected

diagnostic condition prevents reliable control, or if a loss of power occurs, the

servo valve return spring forces the valve to connect the appropriate control

pressure to drain, causing the cylinder to move to the fail-safe position.

Cylinder Position Control

The cylinder position controller adjusts the hydraulic power cylinder position to

match the feedback signal to the demand.

Both the servo position controller and cylinder position controller are monitored to

ensure accurate tracking.

The position controller regulates a pulse width modulated (PWM) drive signal to

the actuator. The drive current to the actuator is regulated, transiently allowing up

to 10 Amps to be provided to move the actuator at its maximum speed and

torque. A steady state current limit becomes active after a period of a few

seconds to protect the actuator and electronics.

The primary demand and redundant demand / feedback input signals are

designed for a (4 to 20) mA control signal. Each input signal is EMC protected.

Discrete outputs are provided for alarm and shut down enunciation. An internal

LED also is illuminated when a fault condition is detected. Cover needs to be

removed to see this LED. The configurable discrete output can be customized to

output a variety of enunciations using the PC Service Tool. All of the discrete

outputs are configurable for normally-open or normally-closed action using the

PC Service Tool.